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/*
* OpenVPN -- An application to securely tunnel IP networks
* over a single TCP/UDP port, with support for SSL/TLS-based
* session authentication and key exchange,
* packet encryption, packet authentication, and
* packet compression.
*
* Copyright (C) 2010-2018 Fox Crypto B.V. <openvpn@fox-it.com>
*
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2
* as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/
/**
* @file
* Control Channel Processor module documentation file.
*/
/**
* @defgroup control_processor Control Channel Processor module
*
* This module controls the setup and maintenance of VPN tunnels and the
* associated security parameters.
*
* @par This module's role
* The Control Channel Processor module lies at the core of OpenVPN's
* activities. It handles the setup of new VPN tunnels, the negotiation
* of data channel security parameters, the managing of active VPN
* tunnels, and finally the cleanup of expired VPN tunnels.
*
* @par State structures
* A large amount of VPN tunnel state information must be stored within an
* OpenVPN process. A wide variety of container structures are used by
* this module for that purpose. Several of these structures are listed
* below, and the function of the first three VPN tunnel state containers
* is described in more detail later.
* - VPN tunnel state containers:
* - \c tls_multi, security parameter state for a single VPN tunnel.
* Contains three instances of the \c tls_session structure.
* - \c tls_session, security parameter state of a single session
* within a VPN tunnel. Contains two instances of the \c key_state
* structure.
* - \c key_state, security parameter state of one TLS and data
* channel %key set.
* - Data channel security parameter containers:
* - \c key_ctx_bi, container for two sets of OpenSSL cipher and/or
* HMAC context (both directions). Contains two instances of the \c
* key_ctx structure.
* - \c key_ctx, container for one set of OpenSSL cipher and/or HMAC
* context (one directions.
* - Key material containers:
* - \c key2, container for two sets of cipher and/or HMAC %key
* material (both directions). Contains two instances of the \c key
* structure.
* - \c key, container for one set of cipher and/or HMAC %key material
* (one direction).
* - \c key_direction_state, ordering of %key material within the \c
* key2.key array.
* - Key method 2 random material containers:
* - \c key_source2, container for both halves of random material used
* for %key method 2. Contains two instances of the \c key_source
* structure.
* - \c key_source, container for one half of random material used for
* %key method 2.
*
* @par The life of a \c tls_multi object
* A \c tls_multi structure contains all the security parameter state
* information related to the control and data channels of one VPN tunnel.
* Its life cycle can be summarized as follows:
* -# Initialization: \c tls_multi_init() and \c
* tls_multi_init_finalize(), which are called (indirectly) from \c
* init_instance() when initializing a new \c context structure.
* - Initializes a \c tls_multi structure.
* - Allocates the three \c tls_session objects contained by the \c
* tls_multi structure, and initializes as appropriate.
* -# Management: \c tls_multi_process() and \c tls_pre_decrypt()
* - If a new session is initiated by the remote peer, then \c
* tls_pre_decrypt() starts the new session negotiation in the
* un-trusted \c tls_session.
* - If the, as yet, un-trusted \c tls_session authenticates
* successfully, then \c tls_multi_process() moves it so as to be
* the active \c tls_session.
* - If an error occurs during processing of a \c key_state object,
* then \c tls_multi_process() cleans up and initializes the
* associated \c tls_session object. If the error occurred in the
* active \c key_state of the active \c tls_session and the
* lame-duck \c key_state of that \c tls_session has not yet
* expired, it is preserved as fallback.
* -# Cleanup: \c tls_multi_free(), which is called (indirectly) from \c
* close_instance() when cleaning up a \c context structure.
* - Cleans up a \c tls_multi structure.
* - Cleans up the three \c tls_session objects contained by the \c
* tls_multi structure.
*
* @par The life of a \c tls_session object
* A \c tls_session structure contains the state information related to an
* active and a lame-duck \c key_state. Its life cycle can be summarized
* as follows:
* -# Initialization: \c tls_session_init()
* - Initializes a \c tls_session structure.
* - Initializes the primary \c key_state by calling \c
* key_state_init().
* -# Renegotiation: \c key_state_soft_reset()
* - Cleans up the old lame-duck \c key_state by calling \c
* key_state_free().
* - Moves the old primary \c key_state to be the new lame-duck \c
* key_state.
* - Initializes a new primary \c key_state by calling \c
* key_state_init().
* -# Cleanup: \c tls_session_free()
* - Cleans up a \c tls_session structure.
* - Cleans up all \c key_state objects associated with the session by
* calling \c key_state_free() for each.
*
* @par The life of a \c key_state object
* A \c key_state structure represents one control and data channel %key
* set. It contains an OpenSSL TLS object that encapsulates the control
* channel, and the data channel security parameters needed by the \link
* data_crypto Data Channel Crypto module\endlink to perform cryptographic
* operations on data channel packets. Its life cycle can be summarized
* as follows:
* -# Initialization: \c key_state_init()
* - Initializes a \c key_state structure.
* - Creates a new OpenSSL TLS object to encapsulate this new control
* channel session.
* - Sets \c key_state.state to \c S_INITIAL.
* - Allocates several internal buffers.
* - Initializes new reliability layer structures for this key set.
* -# Negotiation: \c tls_process()
* - The OpenSSL TLS object negotiates a TLS session between itself
* and the remote peer's TLS object.
* - Key material is generated and exchanged through the TLS session
* between OpenVPN peers.
* - Both peers initialize their data channel cipher and HMAC key
* contexts.
* - On successful negotiation, the \c key_state.state will progress
* from \c S_INITIAL to \c S_ACTIVE and \c S_NORMAL.
* -# Active tunneling: \link data_crypto Data Channel Crypto
* module\endlink
* - Data channel packet to be sent to a remote OpenVPN peer:
* - \c tls_pre_encrypt() loads the security parameters from the \c
* key_state into a \c crypto_options structure.
* - \c openvpn_encrypt() uses the \c crypto_options to an encrypt
* and HMAC sign the data channel packet.
* - Data channel packet received from a remote OpenVPN peer:
* - \c tls_pre_decrypt() loads the security parameters from the \c
* key_state into a \c crypto_options structure.
* - \c openvpn_encrypt() uses the \c crypto_options to
* authenticate and decrypt the data channel packet.
* -# Cleanup: \c key_state_free()
* - Cleans up a \c key_state structure together with its OpenSSL TLS
* object, key material, internal buffers, and reliability layer
* structures.
*
* @par Control functions
* The following two functions drive the Control Channel Processor's
* activities.
* - \c tls_multi_process(), iterates through the \c tls_session objects
* within a given \c tls_multi of a VPN tunnel, and calls \c
* tls_process() for each \c tls_session which is being set up, is
* already active, or is busy expiring.
* - \c tls_process(), performs the Control Channel Processor module's
* core handling of received control channel messages, and generates
* appropriate messages to be sent.
*
* @par Functions which control data channel key generation
* - Key method 1 key exchange functions were removed from OpenVPN 2.5
* - Key method 2 key exchange functions:
* - \c key_method_2_write(), generates and processes key material to
* be sent to the remote OpenVPN peer.
* - \c key_method_2_read(), processes key material received from the
* remote OpenVPN peer.
*/
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